1. Field Of The Invention
The present invention relates to MOS-gated semiconductor devices and, more specifically, MOS-gated semiconductor devices in which the upper regions of the device structures are conductivity modulated by a positive feedback mechanism.
2. Description Of The Related Art
During the last decade, the development of power devices with MOS-gate control has become accepted as the most promising approach for power switching applications. Among MOS-gate power devices, the most commonly used structure is the double-diffused power MOSFET (DMOSFET). The DMOSFET has fast switching characteristics. However, the on-resistance of the power DMOSFET increases very rapidly as the breakdown voltage is increased.
Bipolar current conduction has been utilized to develop devices with lower on-state voltage drop compared to the DMOSFET while still retaining the high input impedance MOS gate which allows simple control circuitry. The IGBT is one such device available commercially which has a lower on-state drop compared to the DMOSFET for high-voltage applications but has slow switching characteristics. The switching speed of the IGBT can only be improved by minority carrier lifetime reduction techniques, and the fastest switching IGBT's reported have turn-off times greater than 100 ns. Another problem with the IGBT is the possibility of latch-up of the parasitic thyristor inherent in the IGBT structure under certain device operating conditions. The current conduction capabilities of other three-terminal MOS-controlled bipolar structures, such as emitter-switched MOS-controlled bipolar transistors, are limited by the high on-resistance of the integrated high-voltage driver DMOSFET.
One structural innovation that has been reported to reduce the on-resistance of the DMOSFET while still retaining its switching speed is the minority carrier injection controlled field effect transistor (MICFET). The device structure of the MICFET, shown in FIG. 1, is similar to that of the DMOSFET, with the addition of a floating P injector region 2 whose potential can be controlled by an integrated vertical driver DMOSFET 4. During the blocking state, with zero potential applied to the gate, a high device breakdown voltage is obtained because the injector region 2 acts as a floating guard ring. When a positive gate bias is applied (for the n-channel structure shown in FIG. 1), current flow occurs via the main DMOSFET 6 at drain voltages below 0.7 volts. At higher drain voltages, the potential of the injector region 2 rises sufficiently to produce injection of minority carriers (holes) 8 into the drift region 10. The injected holes 8 conductivity modulate the JFET region 12 of the main DMOSFET 6, resulting in an increase in current density.
The hole current in the MICFET is provided by the vertical driver DMOSFET 4 integrated into the structure. However, since driver DMOSFET 4 is not conductivity modulated, the on-resistance of driver DMOSFET 4 limits the current conduction capability of the device. Thus, the MICFET shows improvement in current density of only about 20% over the DMOSFET at a forward drop of 2 volts.
Accordingly, a need exists for fast-switching, high voltage MOS-gated power semiconductor device with a low on-resistance at relatively high current levels.